1. Field of the Invention
Embodiments of the present invention relate to optical waveguide interferometric-based hydrophones and, more specifically, to mandrels used in such hydrophones.
2. Description of the Related Art
A Bragg grating is an optical element that is reflective to light having wavelengths within a narrow bandwidth that is centered at a wavelength that is referred to as the Bragg wavelength. Bragg gratings are usually formed by photo-induced periodic modulation of the refractive index of an optical waveguide's core. A pair of Bragg gratings having a common Bragg wavelength and separated by a length of waveguide (e.g. a coiled fiber or coil) can form an interferometer that may be interrogated by sending light of the same Bragg wavelength through the interferometer. Reflections of light from the (partially-transmissive) Bragg gratings are sent back to optical detection equipment through the waveguide. By assessing the phase shift in light coincidently reflected from the two Bragg ratings, the length of the coil can be determined, as is well known.
Optical waveguide interferometers can be deployed in various ways to make the length of the coil (and hence, the phase shifts between coincidentally reflected pulses) dependent on physical parameters. For example, Bragg grating interferometers can be deployed in a number of different ways to make acoustic sensors. Reference, “A Fiber Laser Hydrophone Array,” by D. J. Hill, et al., SPIE Vol. 3860. An optical waveguide hydrophone is typically made by winding a section of an optical waveguide (e.g., an optical fiber) separating a pair of Bragg gratings around a compliant cylindrical mandrel. When acoustic pressure impinges on the mandrel, the mandrel deforms slightly, changing the length of the waveguide separating the Bragg gratings. When forming such an acoustic sensor it is beneficial to tightly wind the optical waveguide (optical fiber) around the compliant cylindrical mandrel, which makes the fiber to follow the response of the mandrel that is designed to respond to acoustic pressure wave. The sensitivity of the sensor is proportional to the number of turns (or wraps), as described below.
One issue with mandrel-based optical waveguide, Bragg grating acoustic sensors is that the Bragg gratings themselves should be protected. Strain on the Bragg gratings can cause an excessive shift in the center frequency of the Bragg wavelength such that the Bragg gratings are no longer highly reflective at the correct wavelength. One way to isolate the Bragg gratings from excessive strain is to locate them within the mandrel itself. This can be accomplished by forming bores through the mandrel, locating a Bragg grating in one bore, wrapping the optical waveguide around the mandrel, and then bringing the optical waveguide through another bore such that the other Bragg grating is located in that bore. By placing the Bragg grating loosely inside the bore will isolate the grating from excessive strain and protect it from physical damages.
The length L of an optical waveguide wrapped on a cylindrical mandrel is about:L≈N·π·dwhere N is the number of turns and d is the outer diameter of the mandrel. In order to have optimum interferometer performances in a system utilizing multiple acoustic sensors (e.g., an array), the length between the two gratings should be nearly identical between devices. However, manufacturing tolerances may lead to significant variations in length between the gratings. For example, when a mandrel is turned on a CNC machine, its outside diameter can vary by about +/−0.001 inch. If the optical waveguide is wrapped around the mandrel 70 times, the wrapped length can vary by as much as 0.14 inch. Furthermore, the process used to produce the Bragg gratings can locate the gratings only within a tolerance of about +/−0.1 inch. Thus, it is difficult to tightly wrap an optical waveguide around a cylindrical mandrel while positioning the Bragg gratings inside the mandrel (which may require a precision of +/−0.040 inch to do).
Therefore, a mandrel that reduces the difficulty of accurately controlling the length of an optical waveguide wrapped around the mandrel and allowing Bragg gratings separated by the length of optical waveguide to be accurately positioned would be useful.